Process for manufacturing recycled pulp from used sanitary goods

ABSTRACT

Provided is a process for recovering pulp fiber from used sanitary goods which contain both pulp fiber and a superabsorbent polymer and producing a recycled pulp which contains little ash and which is reusable for sanitary goods. This process comprises: a calcium treatment step for treating used sanitary goods with a water-soluble calcium compound and thereby conducting the dehydration of a superabsorbent polymer contained in the used sanitary goods; a disintegration step for applying a physical force to the used sanitary goods to disintegrate the used sanitary goods into pulp fiber and other materials; a separation step for separating the pulp fiber from a pulp fiber/other materials mixture which has been formed in the disintegration step; a disinfecting step using a disinfectant; and a citric acid treatment step for subjecting the separated pulp fiber to treatment with an aqueous citric acid solution at an acidic pH.

RELATED APPLICATIONS

The present application is a National Phase of International ApplicationNumber PCT/JP2014/073345 filed Sep. 4, 2014 and claims priority toJapanese Application Number 2013-225937 filed Oct. 30, 2013.

TECHNICAL FIELD

The present invention relates to a process for manufacturing recycledpulp that is reusable for sanitary goods, by recovering pulp fiber fromused sanitary goods which contain pulp fiber and a superabsorbentpolymer.

BACKGROUND ART

There is a trend toward recovery and reuse of pulp from used sanitarygoods. Sanitary goods generally include an absorbent body, the absorbentbody being composed of pulp and a superabsorbent polymer. In order torecover pulp from used sanitary goods, therefore, it is necessary toseparate the pulp and the superabsorbent polymer. However, separation ofsuperabsorbent polymers and pulp that have swelled by absorption ofwater is not easy to accomplish. Separation is carried out by treatmentwith water-soluble calcium compounds such as hydrated lime, calcinedlime and calcium chloride, separating out the superabsorbent polymer bydehydration, sedimentation, dry sorting (for example, air sorting) andsifting. (Patent Literature 1, Patent Literature 2).

Since in the production of pulp, metals such as iron, copper ormanganese in the pulp promote decomposition of oxygen-based bleachingchemicals, resulting in wasteful consumption of the oxygen-basedbleaching chemicals, PTL 3 and PTL 4 disclose methods in which prior tothe bleaching step, the metals are treated with an inorganic acid,organic acid or chelating agent and removed. However, these prior artdocuments do not focus on removal of calcium.

CITATION LIST Patent Literature

-   [Patent Literature 1] Japanese Unexamined Patent Publication No.    2010-84031-   [Patent Literature 2] Japanese Unexamined Patent Publication No.    2004-42038-   [Patent Literature 3] Japanese Unexamined Patent Publication No.    H10-72788-   [Patent Literature 4] Japanese Unexamined Patent Publication No.    2007-270383

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

Pulp recovered by treating used sanitary goods with water-solublecalcium compounds such as hydrated lime, calcined lime or calciumchloride and dehydrating the superabsorbent polymer to separate out thepulp (hereunder referred to as “recovered pulp”) has a highconcentration of ash residue due to calcium, and has been unsuitable forreuse in sanitary goods.

The present invention provides a process for recovering recycled pulpfrom used sanitary goods, which has a low ash content and is reusable insanitary goods.

Means for Solving the Problems

The present inventors have completed this invention upon finding that ifused sanitary goods are treated with a water-soluble calcium compound,the superabsorbent polymer is dehydrated and the pulp separated, and therecovered pulp is treated with an aqueous citric acid solution, it ispossible to effectively obtain recycled pulp with a low ash content,that is reusable in sanitary goods.

Specifically, the invention is a process for manufacturing recycled pulpthat is reusable for sanitary goods, by recovering pulp fiber from usedsanitary goods containing pulp fiber and a superabsorbent polymer, theprocess comprising:

a calcium treatment step in which used sanitary goods are treated with awater-soluble calcium compound to dehydrate the superabsorbent polymerin the used sanitary goods,

a disintegration step in which the used sanitary goods are subjected tophysical force to disintegrate the used sanitary goods into pulp fiberand other materials,

a separation step in which pulp fiber is separated from a mixture ofpulp fiber and other materials generated in the disintegration step,

a disinfection step using a disinfectant, and

a citric acid treatment step in which the separated pulp fiber istreated with an aqueous citric acid solution at an acidic pH.

The invention encompasses the following aspects.

[1] A process for manufacturing recycled pulp that is reusable forsanitary goods, by recovering pulp fiber from used sanitary goodscontaining pulp fiber and a superabsorbent polymer, the processcomprising:

a calcium treatment step in which used sanitary goods are treated with awater-soluble calcium compound to dehydrate the superabsorbent polymerin the used sanitary goods,

a disintegration step in which the used sanitary goods are subjected tophysical force to disintegrate the used sanitary goods into pulp fiberand other materials,

a separation step in which pulp fiber is separated from a mixture ofpulp fiber and other materials generated in the disintegration step,

a disinfection step using a disinfectant, and

a citric acid treatment step in which the separated pulp fiber istreated with an aqueous citric acid solution at an acidic pH.

[2] The process according to claim 1, wherein the pH in the citric acidtreatment step is in the range of 2 to 6.

[3] The process according to [1] or [2], further comprising a rinsingstep.

[4] Recycled pulp with an ash content of less than 4.0% by weight,produced by the process according to any one of [1] to [3].

[5] Recycled pulp according to [4], exhibiting a weakly acidic pH as asolution of 10 g of recycled pulp added to and impregnated with 100 mLof ion-exchanged water.

[6] Recycled pulp according to [4] or [5], wherein the recycled pulp isused in at least one type from among absorbent bodies, tissues andnonwoven fabrics composing sanitary goods.

EFFECT OF THE INVENTION

The recycled pulp produced according to the invention has an ash contentsufficiently low to allow its reuse for sanitary goods.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing an apparatus for measurement ofthe water absorption factor.

FIG. 2 is a flow chart showing a process of manufacturing recycled pulpaccording to one embodiment.

MODE FOR CARRYING OUT THE INVENTION

The present invention relates to a process for manufacturing recycledpulp that is reusable for sanitary goods, by recovering pulp fiber fromused sanitary goods containing pulp fiber and a superabsorbent polymer.

The sanitary goods are not particularly restricted so long as theycontain pulp fiber and a superabsorbent polymer, and examples includedisposable diapers, incontinence pads, urine-absorbing pads, sanitarynapkins and panty liners. Among these, they are preferably incontinencepads or disposable diapers recovered in mass from institutions or thelike, as they require no effort for separation and have relatively largeamounts of pulp.

There are no particular restrictions on the pulp fiber, and examplesinclude fluffy pulp fiber and chemical pulp filaments.

A superabsorbent polymer (SAP) has a three-dimensional network structurewith an appropriately crosslinked water-soluble polymer and thereforeabsorbs a few ten to a few hundred times its weight of water, but it isessentially water-insoluble and the absorbed water does not emerge evenwith some degree of pressure application; examples thereof includestarch-based, acrylic acid-based and amino acid-based particulate orfibrous polymers.

Throughout the present specification, pulp produced by the process ofthe invention will be referred to as “recycled pulp”.

As shown in FIG. 2, a process of manufacturing recycled pulp accordingto one embodiment of the invention comprises:

a calcium treatment step S1 in which used sanitary goods are treatedwith a water-soluble calcium compound to dehydrate the superabsorbentpolymer in the used sanitary goods,

a disintegration step S2 in which the used sanitary goods are subjectedto physical force to disintegrate the used sanitary goods into pulpfiber and other materials, wherein the disintegration step may becarried out after, before or simultaneously with the calcium treatmentstep,

a separation step S3 in which pulp fiber is separated from a mixture ofpulp fiber and other materials generated in the disintegration step,

a disinfection step S4 using a disinfectant, wherein the disinfectionstep may be carried out immediately after the calcium treatment step, orimmediately after the disintegration step, or immediately after theseparation step, or immediately after the citric acid treatment stepdescribed below, or simultaneously with the calcium treatment step, orsimultaneously with the disintegration step, or simultaneously with theseparation step, or simultaneously with the citric acid treatment step,and

a citric acid treatment step S5 in which the separated pulp fiber istreated with an aqueous citric acid solution at an acidic pH, whereinthe citric acid treatment step is carried out after the separation step.

The process of the invention may further comprise a rinsing step ifnecessary.

The process of the invention comprises a calcium treatment step in whichused sanitary goods are treated with a water-soluble calcium compound todehydrate the superabsorbent polymer in the used sanitary goods.

In this step, the superabsorbent polymer that has swelled by absorptionof water in the used sanitary goods is dehydrated by calcium ion.

The superabsorbent polymer has hydrophilic groups (for example, —COO⁻),allowing water molecules to bond to the hydrophilic groups by hydrogenbonding, so that large amounts of water can be absorbed, and when thewater-absorbed superabsorbent polymer is treated with the water-solublecalcium compound, presumably calcium ions bond to the hydrophilic groups(for example, —COO⁻) (forming —COO—Ca—OCO—, for example), breaking thehydrogen bonds between the hydrophilic groups and the water moleculesand releasing the water molecules, so that the superabsorbent polymerbecomes dehydrated.

By dehydrating the superabsorbent polymer, separation between the pulpfiber and superabsorbent polymer becomes easier. When it is attempted todisinfect used sanitary goods that have not been subjected to calciumtreatment, with an aqueous solution containing a disinfectant, thesuperabsorbent polymer absorbs the aqueous solution containing thedisinfectant and temperature efficiency is lowered, but this can beavoided by first dehydrating the superabsorbent polymer by calciumtreatment. When disinfection is carried out with an aqueous solutioncontaining a disinfectant but no water-soluble calcium compound, sincethe superabsorbent polymer absorbs the water of the aqueous solution,unless the superabsorbent polymer is first dehydrated the solidconcentration in the tank increases and mechanical disintegrationoperation becomes difficult, requiring the use of more aqueous solution.

The water-soluble calcium compound to be used in the disintegration stepis not particularly restricted so long as it is a water-soluble calciumcompound and dissolves in water thereby ionizing calcium ion, andexamples include calcium chloride, calcium oxide (calcined lime),calcium hydroxide (hydrated lime) and the like. Calcium chloride andcalcium oxide are preferred among these.

The method of treatment with the water-soluble calcium compound is notparticularly restricted so long as it is a method that allows thesuperabsorbent polymer in the used sanitary goods to contact with thewater-soluble calcium compound, and for example, it may be a method ofdirectly sprinkling the solid water-soluble calcium compound onto theused sanitary goods, or a method of immersing the used sanitary goodsinto an aqueous solution of the water-soluble calcium compound.

The amount of water-soluble calcium compound used in the calciumtreatment step is preferably 4 mol or greater, more preferably 4 to 40mol and even more preferably 5 to 20 mol, per 1 kg of superabsorbentpolymer (based on dry mass). While it is difficult to accuratelydetermine the mass (dry mass) of a superabsorbent polymer from sanitarygoods containing excreted fluid, as a general rule 5 to 15% by weight ofsanitary goods containing excreted fluid corresponds to the mass (drymass) of the superabsorbent polymer. If the amount of water-solublecalcium compound is too low, dehydration of the superabsorbent polymerwill be inadequate. The water-soluble calcium compound will usually beloaded in excess, with the aim of amply ensuring treatment efficiency.

The time for the calcium treatment step is not particularly restrictedso long as it is a sufficient time to allow the calcium ion to be takenup into the superabsorbent polymer.

The treatment time for the calcium treatment step is preferably 10minutes or greater, more preferably 20 minutes to 2 hours and even morepreferably 40 minutes to 90 minutes.

If the treatment time is too short, dehydration of the superabsorbentpolymer may be insufficient. If the treatment time exceeds a certainvalue, the amount of calcium ion taken up into the superabsorbentpolymer will become saturated, and therefore any treatment timeexceeding that value is meaningless.

The temperature of treatment with the aqueous solution of thewater-soluble calcium compound is not particularly restricted so long asit is a temperature at which calcium ion is taken up into thesuperabsorbent polymer, but it will usually be higher than 0° C. andlower than 100° C. Although room temperature is adequate, heating may beperformed to increase the reaction rate. In the case of heating, it ispreferably from room temperature to 60° C., more preferably from roomtemperature to 40° C., and even more preferably from room temperature to30° C.

The amount of aqueous solution used for treatment with the aqueoussolution of the water-soluble calcium compound is not particularlyrestricted so long as it is an amount allowing complete immersion of theused sanitary goods, but it is preferably 3 to 50 kg and more preferably3 to 10 kg to 1 kg of waste-containing used sanitary goods. If theamount of aqueous solution is too small, it may not be possible toeffectively agitate the used sanitary goods in the aqueous solution. Ifthe amount of aqueous solution is too large, the water-soluble calciumcompound may be wasted, increasing treatment costs.

When the used sanitary goods are to be immersed in the aqueous solutionof the water-soluble calcium compound, agitation is not essential but ispreferred.

Even when a solid water-soluble calcium compound is directly sprinkledonto the used sanitary goods, agitation is not essential but ispreferred. If necessary, after a suitable period of time has passedafter directly sprinkling the solid water-soluble calcium compound onthe used sanitary goods, the minimum amount of water necessary to allowagitation may be supplied and agitation carried out.

The process of the invention comprises a disintegration step in which aphysical force is acted on the used sanitary goods to disintegrate theused sanitary goods into pulp fiber and other materials.

Sanitary goods are usually composed of various materials such as pulpfiber, superabsorbent polymers, nonwoven fabrics and synthetic resinfilms. In the disintegration step, the used sanitary goods aredisintegrated into each of these materials. The degree of disintegrationneed only be disintegration allowing at least a portion of the pulpfiber to be recovered, and it does not necessarily need to be completeand instead may be only partial.

The method of applying physical force to the used sanitary goods is notrestricted, and examples include agitation, beating, puncturing,vibration, tearing, cutting, crushing or the like. Among these,agitation in water is preferred. The agitation may be carried out in avessel equipped with a stirrer, such as a washing machine. The agitationconditions are not particularly restricted so long as the sanitary goodsare disintegrated, and for example, the agitation time is preferably 5to 60 minutes, more preferably 10 to 50 minutes and even more preferably20 to 40 minutes.

The disintegration step may be carried out after the calcium treatmentstep, or it may be carried out before the calcium treatment step. Forexample, the used sanitary goods may be torn and the absorbent body andother materials disintegrated, and the disintegrated absorbent body orthe disintegrated absorbent body and other materials may be treated withan aqueous solution of a water-soluble calcium compound. However, whenthe disintegration step is carried out before the calcium treatmentstep, the disintegration step is accomplished without using water. Whenwater or a disinfectant is used in the disintegration step, thedisintegration step is carried out after the calcium treatment step.

In addition, the disintegration step may be carried out separately fromthe calcium treatment step, or the calcium treatment step and thedisintegration step may be carried out as a single step. That is,instead of carrying out the calcium treatment step and thedisintegration step separately, a single calciumtreatment/disintegration step may be provided in which calcium treatmentand disintegration are carried out simultaneously. For example, the usedsanitary goods, water-soluble calcium compound and water may be loadedinto a washing machine, and agitation applied to an extent such that theused sanitary goods disintegrate, thereby allowing simultaneous calciumtreatment and disintegration of the used sanitary goods.

The process of the invention comprises a separation step in which pulpfiber is separated from a mixture of pulp fiber and other materialsgenerated in the disintegration step.

In the separation step, pulp fiber is separated from the mixture of thepulp fiber, dehydrated superabsorbent polymer and other materialsgenerated by disintegration of the used sanitary goods. The method ofseparating the pulp fiber is not limited, and for example, it may be amethod in which the difference in specific gravity of the disintegratedstructural materials (pulp fiber, superabsorbent polymer, plastic, etc.)is utilized for precipitating separation in water, a method in which thedisintegrated structural materials of different sizes are separated bypassing them through a screen with a prescribed mesh, or a method ofseparation with a cyclone-type centrifugal separator.

The process of the invention also comprises a disinfection step using adisinfectant.

The disinfection step may be carried out by treatment of the material tobe disinfected using a disinfectant. For example, it may be carried outby loading the material to be disinfected and the disinfectant into avessel and causing agitation.

The disinfectant to be used in the disinfection step is not particularlyrestricted and may be an aqueous solution dissolving a disinfectant suchas sodium hypochlorite or chlorine dioxide, or ozone water, electrolyticwater (acidic electrolytic water) or the like, among which an aqueoussodium hypochlorite solution is preferred from the viewpoint of economyand general utility.

When using an aqueous solution dissolving a disinfectant, thedisinfectant concentration is not particularly restricted so long as itis a concentration that can yield recycled pulp having the desired levelof hygiene. The preferred concentration for the disinfectant will differdepending on the type of disinfectant, but in the case of sodiumhypochlorite, it is 10 to 1000 ppm by mass, more preferably 30 to 500ppm by mass and even more preferably 50 to 250 ppm by mass. If theconcentration is too low it will not be possible to obtain a sufficientdisinfecting effect, and bacteria and the like may reside in therecycled pulp. If the concentration is too high, not only will thedisinfectant tend to be wasted but the material to be disinfected mayalso be damaged, and problems of safety may arise.

The amount of disinfectant to be used in the disinfection step is notparticularly restricted so long as it is an amount that thoroughly soaksthe material to be disinfected, but it is preferably 3 to 50 kg and morepreferably 3 to 10 kg with respect to 1 kg of the material to bedisinfected. If the amount of disinfectant is too small, it may not bepossible to obtain a sufficient disinfecting effect. If the amount ofdisinfectant is too large, the disinfectant solution or disinfectant maybe wasted and treatment costs may be increased.

The time for the disinfection step is not particularly restricted solong as it is a time that allows recycled pulp to be obtained having thedesired level of hygiene, but it is preferably 10 to 120 minutes, morepreferably 20 to 100 minutes and even more preferably 30 to 80 minutes.

When the disinfection step is carried out in a batch process, thedisinfectant used is discarded upon completion of the disinfection step.

The disinfection step may be carried out immediately after theseparation step, or immediately after the disintegration step, orimmediately after the calcium treatment step, or immediately after thecitric acid treatment step described below. When the disinfection stepis carried out immediately after the separation step, the material to bedisinfected will be pulp fiber, when the disinfection step is carriedout immediately after the disintegration step, the material to bedisinfected will be a mixture of pulp fiber and other materialsgenerated in the disintegration step, when the disinfection step iscarried out immediately after the calcium treatment step, the materialto be disinfected will consist of the used sanitary goods that havecompleted dehydrating treatment, and when the disinfection step iscarried out immediately after the citric acid treatment step, thematerial to be disinfected will be pulp fiber that has completed thecitric acid treatment. When the pulp fiber is to be disinfected, it maybe accomplished by immersing it in the disinfectant and, if necessary,agitated.

The disinfection step may also be carried out simultaneously withanother step, instead of being performed separately.

For example, if the disinfectant is added to the aqueous solution of thewater-soluble calcium compound to be used in the calcium treatment step,the calcium treatment step and the disinfection step can be carried outsimultaneously. That is, instead of carrying out the calcium treatmentstep and the disinfection step separately, a single calciumtreatment/disinfection step may be provided in which calcium treatmentand disinfection step are carried out simultaneously.

Also, instead of carrying out the disintegration step and thedisinfection step separately, there may be provided a singledisintegration/disinfection step in which disintegration anddisinfection are carried out simultaneously. For example, by carryingout disintegration of the used sanitary goods by agitation in adisinfectant, the disintegration step and the disinfection step can becarried out simultaneously.

Also, instead of carrying out the calcium treatment step, disintegrationstep and disinfection step separately, a single calciumtreatment/disintegration/disinfection step may be provided in whichcalcium treatment, disintegration and disinfection step are carried outsimultaneously. For example, if the disinfectant is added to the aqueoussolution of the water-soluble calcium compound to be used in the calciumtreatment step, and the agitation is performed to an extent causingdisintegration of the used sanitary goods, then calcium treatment,disintegration and disinfection can be carried out simultaneously.

The process of the invention comprises a citric acid treatment step inwhich the separated pulp fiber is treated with an aqueous citric acidsolution at an acidic pH.

In the citric acid treatment step, the calcium compounds residing in thepulp fiber are removed.

Treatment with the water-soluble calcium compound in the calciumtreatment step causes adhesion of calcium ions and various calciumcompounds on the surface of the separated pulp fiber. The calciumcompounds adhering to the pulp fiber are not necessarily limited towater-soluble compounds and include insoluble and poorly solublecompounds, which cannot be removed by rinsing alone. Citric acid forms achelate with calcium resulting in water-soluble calcium citrate, and cantherefore effectively dissolve and remove insoluble or poorly solublecalcium compounds adhering to the surface of the pulp fiber. Sincecitric acid can also form chelates with metals other than calcium, wheninsoluble or poorly soluble metal compounds other than calcium compoundsare adhering to the surface of the pulp fiber, it is possible todissolve and remove not only the calcium compounds but also theinsoluble or poorly soluble metal compounds other than the calciumcompounds. As a result, it is possible to reduce the ash content of theobtained recycled pulp.

The following advantages are afforded by using citric acid.

Firstly, citric acid is acidic and thus, depending on the set conditionsincluding the washing step, it is possible to control the pH of therecycled pulp to a weakly acidic range which is favorable for skin.

Secondly, because citric acid is not a hazardous substance for the humanbody, it is highly safe even if the citric acid remains in the obtainedrecycled pulp.

Thirdly, since citric acid is a mildly weak acid compared to the acidused in the pulp purification, it can reduce damage to the obtainedrecycled pulp.

Fourthly, citric acid can be obtained relatively cheaply, therebylowering cost for recovery and regeneration.

Fifthly, citric acid is odorless and therefore does not impair theworking environment.

Sixthly, it does not require major equipment investment, allowingexisting facilities to be employed.

The concentration of the aqueous citric acid solution used in the citricacid treatment step is not particularly restricted so long as it is aconcentration that allows adjustment to the prescribed pH and canadequately reduce the ash content, but it is preferably 5 to 250 mol/m³.If the concentration is too low, it will not be possible to adequatelyreduce the ash content. If the concentration is too high, waste of thecitric acid may result, running up treatment costs.

The amount of aqueous citric acid solution to be used in the acidtreatment step is not particularly restricted so long as it is an amountthat thoroughly soaks the material to be treated, but it is preferably 3to 50 kg and more preferably 3 to 10 kg with respect to 1 kg of thematerial to be treated. If the amount of aqueous solution is too low, itwill not be possible to adequately reduce the ash content. If the amountof aqueous solution is too high, waste of the citric acid may result,increasing the treatment cost.

The citric acid treatment step is carried out at an acidic pH. That is,the acid treatment is carried out at a pH of lower than 7. When analkaline calcium compound has been used in the disintegration step,alkaline calcium compounds often reside in the pulp fiber that issupplied to the acid treatment step, and addition of this pulp fiber tothe aqueous citric acid solution will sometimes alter the pH of theaqueous citric acid solution. When the pH of the aqueous citric acidsolution differs from before and after addition of the pulp fiber, thepH in the acid treatment step in such a case refers to the pH of theaqueous citric acid solution after addition of the pulp fiber.

For adjustment of the pH, for example, the pulp fiber and water areplaced in a treatment tank, successively adding citric acid whilestirring, and the citric acid addition is halted once the pH of thesolution in the treatment tank reaches the prescribed pH.

The pH in the citric acid treatment step is preferably 2 to 6, morepreferably 2 to 4.5, even more preferably 2 to 3.5 and yet morepreferably 2 to 3.

If the pH is too low, the water absorption factor of the obtainedrecycled pulp may be lowered. If only the ash content is a concern, theobtained recycled pulp will be reusable as sanitary goods even with alow pH, but when the water absorption factor is also a consideration,the pH is preferably 2 or higher. The reason for a low water absorptionfactor of the obtained recycled pulp when the pH is too low is notclearly understood, but may be because the pulp fiber itself undergoes atransformation.

If the pH is too high, the ash content of the obtained recycled pulpwill tend to increase. So long as treatment is at a pH of below 7, theash content in the pulp fiber that has passed through the disintegrationstep can be reduced to a level allowing reuse as sanitary goods, butconsidering that the standard for sanitary product materials establishedby the Japan Hygiene Products Industry Association is an ash content ofno higher than 0.65%, the pH is preferably 3.5 or lower.

Also, if the pH is too high, the water absorption factor of the obtainedrecycled pulp will tend be lowered. The reason for which the waterabsorption factor of the recycled pulp cannot be adequately restoredwith a high pH is not completely understood, but it is possible thatinorganic materials such as insoluble or poorly soluble calciumcompounds adhering onto the surface of the pulp fiber lower thehydrophilicity of the pulp fiber and reduce the water absorption factor,and when the pH is high it is not possible to thoroughly remove theinorganic materials such as insoluble or poorly soluble calciumcompounds adhering to the surface of the pulp fiber, resulting ininsufficient restoration of the water absorption factor. From theviewpoint of the water absorption factor, the pH is preferably 4.5 orlower.

The time for the citric acid treatment step is not particularlyrestricted so long as it is a time allowing the ash content to belowered, but it is preferably 1 to 80 minutes, more preferably 2 to 40minutes and even more preferably 4 to 20 minutes. If the treatment timeis too short, it will not be possible to adequately reduce the ashcontent. If the treatment time is too long, treatment costs mayincrease.

The temperature in the citric acid treatment step is not particularlyrestricted so long as it is a temperature that allows the ash content tobe lowered. The aqueous citric acid solution may be heated if necessary,but alternatively room temperature may be maintained.

While agitation of the aqueous citric acid solution is not essential inthe citric acid treatment step, a moderate degree of agitation ispreferred.

The citric acid treatment step is carried out at a later stage than theseparation step. That is, citric acid treatment is carried out after thepulp fiber has been separated from the dehydrated superabsorbentpolymer. If citric acid treatment is carried out at a stage in which thedehydrated superabsorbent polymer has not yet been separated, then thesuperabsorbent polymer will reabsorb water and the treatment efficiencywill be reduced.

The citric acid-treated pulp fiber is preferably rinsed with water inthe rinsing step.

The method for rinsing the pulp fiber is not particularly restricted,and for example, it may be accomplished by simple water rinsing. Waterrinsing can be carried out in a batch system or a semi-batch system, oreven in a circulating system. When carried out in a batch system, thewater rinsing may be performed using a washing machine, for example.

The rinsing conditions are not particularly restricted so long as theyallow adequate removal of substances other than the pulp fiber, and forexample, the rinsing time in a batch system is preferably 1 to 80minutes, more preferably 2 to 40 minutes and even more preferably 4 to20 minutes. When carried out in a batch system, the amount of water usedis preferably 3 to 50 kg and more preferably 3 to 10 kg with respect to1 kg of the material to be rinsed.

The rinsed pulp fiber may then be dehydrated in a dehydration step, ifnecessary.

The method for dehydrating the rinsed pulp fiber is not particularlyrestricted, and for example, it may be accomplished by dehydrating therinsed pulp fiber with a dehydrator such as a centrifugal separator.

The dehydrating conditions are not particularly restricted so long asthey allow the moisture content to be lowered to the target value, butfor example, the dehydrating time is preferably 1 to 10 minutes, morepreferably 2 to 8 minutes and even more preferably 3 to 6 minutes.

The rinsing step and dehydration step may be carried out once each, orthey may be repeated several times in alternate fashion.

The pulp fiber that has been subjected to citric acid treatment and, ifnecessary, rinsing and dehydration, is then dried in a drying step ifnecessary.

The method for drying the pulp fiber is not particularly restricted, andfor example, it may be accomplished using a dryer such as a hot airdrier.

The drying conditions are not particularly restricted so long as theyallow adequate drying of the pulp fiber, and for example, the dryingtemperature is preferably 80 to 200° C., more preferably 90 to 150° C.and even more preferably 100 to 120° C. The drying time is preferably 10minutes to 30 hours and more preferably 20 to 60 minutes.

The moisture content of the dried pulp fiber is preferably 5 to 13% byweight, more preferably 6 to 12% by weight and even more preferably 7 to11% by weight. If the moisture content is too low, the hydrogen bondsmay become too strong resulting in excessive hardness, while converselyif the moisture content is too high, mold and the like may be generated.

The moisture content of the pulp fiber is measured in the followingmanner. The measurement is conducted in an atmosphere of 20° C.±1° C.

(1) The mass A (g) of the vessel into which the sample to be measuredwill be placed (a non-covered vessel) is measured.

(2) Approximately 5 g of the sample to be measured is prepared andplaced in the vessel whose mass was measured in (1), and the mass B (g)of the sample-containing vessel is measured.

(3) The sample-containing vessel is placed for 2 hours in an oven set ata temperature of 105° C.±3° C.

(4) The sample-containing vessel is removed from the oven and placed for30 minutes in a desiccator (desiccant: colored silica gel-containingsubstance).

(5) The sample-containing vessel is removed from the desiccator and themass C (g) is measured.

(6) The moisture content (%) is calculated by the following formula.Moisture content (%)=(B−C)/(C−A)×100

The dried pulp fiber is preferably worked into a form such as a sheet,roll or mass, that can be easily managed in the production equipment forsanitary goods, and reused.

In the process of the invention, a rinsing step and/or a dehydrationstep may be provided immediately after the calcium treatment step. Arinsing step and/or a dehydration step may also be provided immediatelyafter the disintegration step. A rinsing step and/or a dehydration stepmay also be provided immediately after the separation step, as well. Inaddition, a rinsing step and/or a dehydration step may be providedimmediately after the disinfection step. The rinsing step anddehydration step may be carried out in the same manner as the rinsingstep and dehydration step following the acid treatment step.

The process of the invention includes, but is not limited to, thefollowing modes.

(a) Calcium treatment step→disintegration step→separationstep→disinfection step→citric acid treatment step

(b) Calcium treatment step→disintegration step→separationstep→disinfection step→citric acid treatment step→dehydrationstep→drying step

(c) Calcium treatment step→disintegration step→separationstep→disinfection step→citric acid treatment step→rinsingstep→dehydration step→drying step

(d) Calcium treatment step→disintegration step→separationstep→disinfection step→dehydration step →citric acid treatmentstep→rinsing step→dehydration step→drying step

(e) Calcium treatment step→disintegration step→separationstep→disinfection step→rinsing step→dehydration step→citric acidtreatment step→rinsing step→dehydration step→drying step

(f) Calcium treatment step→disintegration step→separationstep→dehydration step→disinfection step →dehydration step→citric acidtreatment step→rinsing step→dehydration step→drying step

(g) Calcium treatment step→disintegration step→rinsing step→separationstep→dehydration step→disinfection step→dehydration step→citric acidtreatment step→rinsing step→dehydration step→drying step

(h) Calcium treatment step→rinsing step→disintegration step→separationstep→dehydration step →disinfection step→rinsing step→dehydrationstep→citric acid treatment step→rinsing step→dehydration step→dryingstep

(i) Calcium treatment step→rinsing step→disintegration step→separationstep→rinsing step→dehydration step→disinfection step→rinsingstep→dehydration step→citric acid treatment step→rinsingstep→dehydration step→drying step

(j) Calcium treatment step→disintegration/disinfection step→separationstep→citric acid treatment step

(k) Calcium treatment step→disintegration/disinfection step→rinsingstep→separation step→dehydration step→citric acid treatment step→rinsingstep→dehydration step→drying step

(l) Calcium treatment/disinfection step→disintegration step→separationstep→citric acid treatment step

(m) Calcium treatment/disinfection step→rinsing step→disintegrationstep→separation step→rinsing step→dehydration step→citric acid treatmentstep→rinsing step→dehydration step→drying step

(n) Calcium treatment/disintegration/disinfection step→separationstep→citric acid treatment step

(o) Calcium treatment/disintegration/disinfection step→rinsingstep→separation step→dehydration step→citric acid treatment step→rinsingstep→dehydration step→drying step

(p) Disintegration step→calcium treatment step→separationstep→disinfection step→citric acid treatment step

(q) Disintegration step→calcium treatment step→rinsing step→separationstep→disinfection step→dehydration step→citric acid treatmentstep→rinsing step→dehydration step→drying step

Recycled pulp obtained by the process of the invention preferably has anash content of less than 4.0% by weight. More preferably, it is recycledpulp that is reusable in a sanitary napkin, having an ash content of nogreater than 0.65% by weight.

The method of measuring the ash content is described below.

Recycled pulp obtained by the process of the invention preferably has awater absorption factor of 7.0 g/g.

The method of measuring the water absorption factor is also describedbelow.

Recycled pulp obtained by the process of the invention preferablyexhibits a weakly acidic pH as a solution of 10 g of the recycled pulpadded to and impregnated with 100 mL of ion-exchanged water. Here,“weakly acidic” means a pH in the range of about 3 to 6.

Recycled pulp obtained by the process of the invention is preferablyused in at least one from among absorbent bodies, tissues and nonwovenfabrics composing sanitary goods.

EXAMPLES

In the examples, recovered pulp produced by the method described inJapanese Unexamined Patent Publication No. 2010-84031 was subjected tocitric acid treatment, and the effect of the citric acid treatment wasconfirmed. The recovered pulp used had an ash content of 8.51% by weightand a water absorption factor of 6.0 g/g. Upon component analysis of theash, the elements composing the ash were found to be 93.055 mol % Ca,6.046 mol % Si, 0.535 mol % K, 0.179 mol % Fe, 0.125 mol % Sr and 0.059mol % Zn. Ca components were found to occupy the major portion of theash. Moreover, X-ray analysis of the Ca component indicated presence informs such as CaO, CaCO₃ and Ca(OH)₂.

A 15 g portion of the recovered pulp was placed in a 2 liter beakercontaining 750 g of citric acid solution at different concentrations,and a stirrer (MAZELA Z-1210 by Eyela) was used for stirring for 10minutes at 600 rpm. The pH of the supernatant liquid in the beaker wasmeasured with a pH meter (Model Twin pH AS-212 by Horiba). Drainage wasperformed using a 250-mesh net. The recovered pulp was returned to thebeaker and subjected to water rinsing for 10 minutes with 750 g ofion-exchanged water. Drainage was performed using a 250-mesh net. Dryingwas carried out for 24 hours with a hot air drier at 105° C. to obtainrecycled pulp.

The ash content and water absorption factor of the obtained recycledpulp were measured. The measuring methods for the ash content and waterabsorption factor were as described below.

The pH, ash content and water absorption factor values are shown inTable 1.

[Ash Content]

The ash content (% by weight) was measured according to “5. Ash contenttest method” in accordance with the Sanitary Material Standards. Thespecific procedure was as follows.

A platinum, quartz or magnetic crucible is strongly preheated at 500 to550° C. for 1 hour, and after standing to cool, the mass W₀ is preciselymeasured. Separately, in addition to the reference, 2 to 4 g of sampleis taken and placed in the crucible, the mass W₁ is precisely measured,removing or displacing the cover of the crucible if necessary, andgentle heating is preferred first, followed by gradual increase in thetemperature to strong heating at 500 to 550° C. for 4 hours or longer,ashing it until no more carbides remain. After being allowed to cool,the mass is precisely measured. The residue is again ashed, and afterbeing allowed to cool, the mass is precisely measured, and ashing,cooling and mass measurement are repeated until a constant mass isreached. When carbides remain and no constant mass is reached by usingthis method, hot water is added to produce leaching, filtration isperformed using quantitative analysis filter paper, and the residue isstrongly heated until no carbides are left with the filter paper or withthe impurities on the filter paper. After adding the filtrate thereto,the mixture is evaporated to dryness and strongly heated. It is allowedto cool, and the mass is precisely measured. When carbides remain evenwith this method, a small amount of ethanol is added to wet it, thecarbides are broken up with a glass rod, the glass rod is washed with asmall amount of ethanol, and after evaporation while monitoring ethanol,the same procedure as before is repeated. The cooling is accomplishedwith a desiccator containing silica gel. Upon reaching a constant mass,the mass is recorded as W₂. The ash content (% by weight) is calculatedby the following formula.Ash content (% by weight)=(W ₂ −W ₀)/(W ₁ −W ₀)×100[Water Absorption Factor]

The water absorption factor (g/g) was measured by the “D/W” method(Demand Wettability method).

For the measurement there was used a Demand Wettability device(hereunder abbreviated as “D/W device”) by Taiyo Create Co., Ltd. FIG. 1shows an overview of a D/W device. In FIG. 1, 1 is a burette, 2 is a0.9% sodium chloride aqueous solution, 3 is a liquid outlet, 4 is asupport plate, 5 is a nylon net, 6 is an acrylic cylinder, 7 is pulp, 8is a deadweight, 9 is a cock, 10 is an air inlet tube, 11 is a valve and12 is a rubber stopper.

The specific procedure for the measurement was as follows.

(1) A 0.9% sodium chloride aqueous solution 2 is introduced through theburette 1 of the D/W device.

(2) With one drop discharged from the liquid outlet 3, a 10 cm-square250 mesh nylon net (NBC Meshtec, Inc.) 5 is placed on the support plate4.

(3) The liquid outlet 3 and center are aligned, and the acrylic cylinder6 with an inner diameter of 33 mm is placed thereon and filled with aprecisely weighed 1.00 g portion of pulp.

(4) A 200 g deadweight 8 matching the inner diameter of the cylinder isplaced on the pulp 7.

(5) The cock 9 is opened, and the stopwatch is started simultaneouslywhen foam begins to exit.

(6) After 60 seconds, the cock 9 is closed and the mass A (g) of thepulp 7 in the cylinder is measured.

(7) The water absorption factor (g/g) is calculated by the followingformula.Water absorption factor=(A−1.00)/1.00[Table 1]

TABLE 1 Ash content Water absorption Ratio of water absorption factor (%by factor with respect to virgin pulp No. Material pH weight) (g/g) (%)1 Citric acid-treated product 1.94 0.09 6.2 84 2 Citric acid-treatedproduct 2.95 0.30 7.3 99 3 Citric acid-treated product 3.52 0.82 7.2 974 Citric acid-treated product 4.24 1.40 7.2 97 5 Citric acid-treatedproduct 5.34 1.90 7.0 95 6 Citric acid-treated product 9.02 4.10 6.9 937 Water-rinsed product 10.29 4.00 6.8 92 (no citric acid used) 8Untreated recycled pulp — 8.51 6.0 81 9 Virgin pulp — 0.18 7.4 100

As is seen from the results in Table 1, citric acid treatment with thepH in an acidic range yielded a product which had a low ash content andwas reusable for sanitary goods.

Moreover, citric acid treatment in an acidic range of pH 2 or greateryielded a product having a water absorption factor of 7.0 g/g orgreater, and recovery was seen to a water absorption factor of 95% orgreater with respect to the virgin pulp (unused pulp product). Althoughthe water absorption factor was of a level suitable for reusable forsanitary goods with treatment at pH 1.94, the water absorption factortended to decrease. The reason for this is unclear but is believed to bethat the pulp itself may have undergone a transformation.

INDUSTRIAL APPLICABILITY

Recycled pulp obtained by the process of the invention can be reused insanitary goods. In particular, it can be used in absorbent bodies,tissues or nonwoven fabrics composing sanitary goods.

REFERENCE SIGNS LIST

-   1 Burette-   2 0.9% Sodium chloride aqueous solution-   3 Liquid outlet-   4 Support plate-   5 Nylon net-   6 Acrylic cylinder-   7 Pulp-   8 Deadweight-   9 Cock-   10 Air inlet tube-   11 Valve-   12 Rubber stopper

The invention claimed is:
 1. A process for manufacturing recycled pulpthat is reusable for sanitary goods, by recovering pulp fiber from usedsanitary goods containing pulp fiber and a superabsorbent polymer, theprocess comprising: a calcium treatment step in which used sanitarygoods are treated with a water-soluble calcium compound to dehydrate thesuperabsorbent polymer in the used sanitary goods, a disintegration stepin which the used sanitary goods are subjected to physical force todisintegrate the used sanitary goods into pulp fiber and othermaterials, a separation step in which pulp fiber is separated from amixture of pulp fiber and other materials generated in thedisintegration step, a disinfection step using a disinfectant, and acitric acid treatment step in which the separated pulp fiber is treatedwith an aqueous citric acid solution at an acidic pH.
 2. The processaccording to claim 1, wherein the pH in the citric acid treatment stepis in a range of 2 to
 6. 3. The process according to claim 1, furthercomprising a rinsing step carried out after the citric acid treatmentstep.
 4. The process according to claim 1, wherein the citric acidtreatment step removes calcium compounds and other metal compoundsresiding in the pulp fiber by forming chelates of citric acid withcalcium and other metals.
 5. A process of manufacturing recycled pulpthat is reusable for sanitary goods, by recovering pulp fiber from usedsanitary goods containing pulp fiber and a superabsorbent polymer, theprocess comprising: treating used sanitary goods with a water-solublecalcium compound to dehydrate the superabsorbent polymer in the usedsanitary goods, disintegrating the used sanitary goods by physical forceinto pulp fiber and other materials, separating the pulp fiber from amixture of the pulp fiber and the other materials generated in thedisintegrating, disinfecting with a disinfectant, and treating theseparated pulp fiber with an aqueous citric acid solution at an acidicpH, wherein the disintegrating is carried out after, before orsimultaneously with the treating with the water-soluble calciumcompound, the separating is carried out after the disintegrating, thedisinfecting is carried out immediately after the calcium treatmentstep, or immediately after the disintegration step, or immediately afterthe separation step, or immediately after the citric acid treatmentstep, or simultaneously with the calcium treatment step, orsimultaneously with the disintegration step, or simultaneously with theseparation step, or simultaneously with the citric acid treatment step,and the treating with the aqueous citric acid solution is carried outafter the separating.
 6. The process according to claim 5, wherein thepH in the citric acid treatment step is in a range of 2 to
 6. 7. Theprocess according to claim 5, further comprising a rinsing step carriedout after the citric acid treatment step.
 8. The process according toclaim 5, wherein the citric acid treatment step removes calciumcompounds and other metal compounds residing in the pulp fiber byforming chelates of citric acid with calcium and other metals.